1. Field of the Invention
The invention relates to a method of controlling the continuity of a fluidized bed gas phase reactor by examining signal complexity. Specifically, the invention relates to a method of detecting sheeting in the reactor by employing non-linear dynamics to evaluate signal complexity and determine reactor continuity. The invention also relates to controlling the reactor continuity by determining a sheeting precursor state and allowing a counter measure to be applied to prevent sheeting occurrences.
2. Related Art
Recirculating fluid-bed reactors are particularly advantageous due to their uniform composition and temperature, ease of sampling and intensive mixing. Ideal reactor continuity requires stable and high production rates, an absence of sheeting occurrences (see U.S. Pat. Nos. 5,436,304 and 5,405,922, which are incorporated herein by reference), and consequently constant production rate. In a sheeting event, maintenance of fluid-bed reactors involves a complete shutdown that translates directly into lost production time. Unfortunately, methods are not known to prevent such catastrophic events.
Reactor operability results from a triad of intercalated parts: catalyst and process chemistry, surface and physical chemistry, and reaction engineering. The latter comprises catalyst delivery systems, particle growth, heat generation and removal, particle morphology, fluidization behavior, condensing mode effects, and process control. Of these factors, efficient removal of heat generated during reactor operation that exceeds rates of heat generation is the crux of understanding and maintaining reactor continuity.
Heat transfer is efficient provided the reaction environment is tailored to provide an acceptably wide thermal stability window at macroscale (whole system), microscale (intra-particle) and mesoscale (inter-particle) levels of operation. To completely control heat transfer, basic principles must be understood. It is widely known that heat transfer results from either conductive or convective mechanisms. This is described in terms of thermal conductivity and convective heat transfer coefficients. These variables are used to derive a Nusselt number (Nu), which has been correlated to single drops of evaporating liquids. It has generally been assumed that the same correlation applies to multi-phase gas-solid flow, however, the role of particle-particle interactions is neglected (mesoscale level). Ignoring the contribution suggests that the correlation is only valid for highly dilute systems. Recently, several reports on the multi-phase heat transfer process based on experimental and theoretical principles have emerged.
Despite the growing interest in the Nusselt number, recent computational fluid dynamics (CFD) studies point to the importance of particle-particle interactions in gas-phase polyethylene polymerization. Results of these studies indicate that a large temperature differential exists between small and larger particles and that inter-particle effects are more influential than an intra-particle gradient. This means that if two particles of approximately the same size make physical contact, a hot spot forms between them. Additionally, if small, highly active particles are shielded from the gas flow without any contact, rapid overheating of the particles occurs. Isolated particles are predicted to be thermally stable provided the reaction is at a constant polymerization rate. It has also been reported that physical contact between small, hot particles and larger, relatively cool particles aids in avoiding overheating. This effect is attributed to the minor role of thermal conduction and convective heat transfer between particles.
The particle surface of a healthy reactor wall is constantly renewed, which is largely determined by the particle residence time. If the particle residence time at the wall is short, then kinetic energy is high and a small adiabatic temperature rise is observed. Thus, fluctuations in heat-flux measurements indicate the degree of particle mixing or residence time at the reactor wall. Noteworthy, steady-state conditions for an individual particle is rapid and occurs within 0.1 seconds or less. Short residence times produce high heat-transfer coefficients and lower temperatures at the wall. As layers of particles accrete to form polymer sheets, the heat-transfer coefficient decreases. Consequently, excess temperatures result in particle fusion and melting, thereby producing polymer sheets. Following this, disruption in fluidization patterns is generally evident, such as, for example, catalyst feed interruption, plugging of the product discharge system, and the occurrence of the sheets (fused agglomerates) in the product.
Maintaining constant and consistent fluidization in a reactor is critical to high throughout. Fluidized bulk density measurements indicate bed-level oscillations, bubbles and slugs. Slugs may also be formed due to the coalescence of bubbles, in particular where there is a high gas/solid ratio. As pressure decreases, the existing gas expands and forms bubbles. Bubbles of gas increase in size and then coalesce to form gas plugs that separate the solid emulsion phase into slugs. The occurrence of slug flow leads to large variations in mass-flow-rates and a decrease in pressure in the reactor. The large amplitude waves move at a velocity less than the mixture velocity.
U.S. Pat. No. 5,148,405, which is incorporated herein by reference, describes the use of acoustic emission to measure slug flow in a multiphase flow pipeline. In a pipeline, disruptions in flow result from gravitational forces, thereby causing stratified unstable waves to grow on the gas/liquid interface that eventually bridge a pipe and form slugs.
Many advantages are afforded by acoustic emissions measurements, namely, real-time information and quantitative and qualitative process control. Acoustic emission is a non-invasive technique that involves either active or passive detection to measure energy in the form of vibrational waves. In general, acoustics refer to the generation, transmission and reception of energy, which can pass through gases, liquids and solids.
Pressure in a reactor is often monitored to indicate indirectly the state of fluidization in the system as a whole by detecting bed-flow oscillations. Pressure differentials are commonly measured with pressure taps. Pressure differentials provide a qualitative measure of the reactor operability and, thus, do not predict or allow prevention of major continuity disturbances. An analysis technique that functions on-line in a manner such that precursors of sheeting states are identified in real-time has not been described.
Because many variables in a reactor system effect non-linear response, use of non-linear models to control the chemical processes resulting in such non-linear effects are recognized in the art. For example, U.S. Pat. No. 6,263,355, which is incorporated herein by reference, describes a rapid noise filter that minimizes spurious control events by removing noise in a sensor or controller output signal. U.S. Pat. No. 6,122,557, which is incorporated herein by reference, teaches a method for controlling a chemical reactor, preferably the pressure, using a feed-forward subroutine for calculating parametric balances responsive to multivariable inputs which takes advantage of a rapid noise filtering subroutine.
The present invention employs non-linear analytical models derived from a continuous reactor in determining the onset and presence of sheeting. Thus, the present invention provides a cost effective and efficient method to evaluate reactor operation in a fluidized bed reactor in order to control major continuity disturbances in the reactor, in. particular, sheeting events. It is these aspects of evaluation, analysis and control of reactor continuity that are addressed herein.